Highly-active, finely divided super-dry silicon dioxide

ABSTRACT

A highly active superdry silica aerogel free of adsorbed water and hydrogen-bonded silanol groups the infrared spectrum of which is characterized by a single narrow intense band at about 3700 cm.sup. -1  within the range between 4000 and 2000 cm.sup. -1  indicating the presence of free silanol groups and which spectrum is further characterized by the absence of bonds indicating the presence of adsorbed water and hydrogen-bonded silanol groups, the said silica aerogel having been produced by subjecting a pyrolytically produced silica aerogel containing adsorbed water and both free and hydrogen-bonded silanol groups to treatment in a fluidized bed at a temperature between about 700° and 1000° C and at a pressure between 2 torrs and atmospheric pressure for a period between 1 second and a few minutes through which fluidized bed a stream of a dried gas that had been preheated to a temperature between 100° and 300° C was passed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of our application Ser. No. 434,577,filed Jan. 18, 1974, now abandoned, which is a division of applicationSer. No. 175,382, filed Aug. 26, 1971, that issued on Jan. 7, 1975, asU.S. Pat. No. 3,859,420, application Ser. No. 175,382 is itself acontinuation of our abandoned application Ser. No. 886,413, filed Dec.18, 1969.

BACKGROUND OF THE INVENTION

The invention relates to a process for making a super-dry finelydispersed silicon dioxide by heating silicon dioxide, particularly suchsilicon dioxide as is obtained in a pyrogenic process of production.

The specific properties of finely dispersed silicon dioxide which isformed by the hydrolytic cleavage of silicon tetrachloride in a flameare largely due to the process of making the material and areparticularly characterized by the small size of the primary particles,the desirable particle size distribution and the structure of itssurface. These properties distinguish this type of silicon dioxide whichis also referred to as a fume silica or silica aerogel from other finelydispersed silicic acids such as are produced by a wet process.

The manner of making pyrogenic silicon dioxide results in a surfacestructure which is formed essentially by three types of so-calledsurface hydroxyl groups which are simultaneously present in all silicondioxide aerogels. These three types of hydroxyl groups are thefollowing:

1. Silanol groups which are present at the surface and which, because ofthe spacing from other silanol groups and their limited action range,cannot enter into any interaction and therefore may be called isolatedor "free silanol groups."

2. Silanol groups of the same type as described at (1) which, however,are so closely spaced that they can interact together by formation ofhydrogen bridges and which may therefore be described as"hydrogen-bonded silanol groups," and are referred to herein simply asbonded silanol groups, and

3. the hydroxyl groups of surface-adsorbed water present in the silicicacid aerogels.

The properties of the silicon dioxide aerogels are not determined by thetotality of all its hydroxyl groups nor, as can be shown experimentally,by the totality of all silanol groups; rather, each of the threedifferent hydroxyl types has its share in the particular properties ofthe product.

For industrial purposes a filler is generally preferred which consistsof a highly pure and dry silicon dioxide which, in addition to silanolgroups, still includes hydroxyl groups. On the other hand, as adsorbentor also as a filler for specific purposes, a silicon dioxide ispreferred which is as free as possible of hydroxyl groups and closelyspaced silanol groups.

When making a finely dispersed silicon dioxide in the flame hydrolysisprocess, it is customary to effect the precipitation of the finelydispersed oxides at temperatures where undesirable condensation of theproducts formed during the reaction, such as water or aqueoushydrochloric acid, is avoided. The final products, however, stillinclude certain additives depending on the type of starting products andthe thermic oxidation or hydrolysis. For instance, if the process startsfrom silicon tetrachloride, a product is obtained which still retainscertain amounts of hydrogen chloride as determined by the highadsorption properties of the formed silicic acid aerosol. In order toremove the hydrogen chloride, it is well known to subject the finelydispersed oxides to a heat treatment at temperatures of about 200° to500° C. and, in any case, below red heat. This treatment may be effectedimmediately after precipitation. Another method is to subject the oxidesto a deacidification by means of superheated steam at similar or alsolower temperatures in a unidirectional current or countercurrent. Inthis case, a good deacidification is obtained, but a certain residualhumidity cannot be eliminated from the aerogel.

One of the most important properties of the silicon dioxide aerogel isits thickening action in which the adsorbed humidity, that is adsorbedwater, is important. It has been found in this connection that ananhydrous silicon dioxide aerogel has a better thickening action incarbon tetrachloride than a silicon dioxide aerogel which still contains1 to 2% water. The products thus obtained usually meet the requirements.However, there are special uses for which absolutely dry products arenecessary.

Such absolutely dry silicon dioxide aerogel can be obtained by a vacuumtreatment at specific conditions such as an increased temperature orwith the aid of isothermal distillation. However, the product will stillretain a more or less substantial amount of "bonded silanol groups."

Scientific investigations have shown that silicon dioxide aerogel whichwas obtained by pyrolitic decomposition of, for instance, silicontetrachloride to a silicon dioxide aerogel with subsequent coagulationand after pressure shaping into thin laminae, this silicon dioxide notonly loses the adsorbed water while being heated to 800° to 1000° C.through extended periods such as 8 hours at a reduced pressure of forinstance 10.sup.⁻³ Torr, but also releases some of its silanol groups bycondensation to water. This process can be traced by infraredspectroscopy since the individual hydroxyl types result in differentband spectra. In this connection it is of particular interest that theso-called "free silanol groups" which are visible in the infraredspectrum as a well-defined band are retained, at least up to a certainextent, at this temperature. At less extended heating periods, the"bonded" silanol groups are likewise retained to a substantial extentsince the completeness of the condensation of these groups depends bothon the temperature and the time of treatment.

All the described processes used for making absolutely dry productshave, however, the shortcoming that they either do not lead to productswhich are free of "bonded" silanol groups or, as in case of thelast-mentioned investigations, cannot be used for mass production and,besides, also result in products which no longer have the loosestructure of the starting product and are therefore technically useless.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processfor making a finely dispersed absolutely dry silicon dioxide by heating,particularly of pyrogenically obtained, silicon dioxide which results ina product which is entirely free of adsorbed water and of closely spacedsilanol groups while, on the other hand, the remaining free silanolgroups have not been affected and wherein the loose structure of thestarting product is at least fully retained.

This is accomplished by subjecting the silicon dioxide to a current of adry gas in a fluidizing bed at a temperature from about 700° to about1000° C. and at normal pressure for a period from a few seconds to a fewminutes.

The invention embraces also the product obtained by this process, whichis characterized in that it retains the more widely spaced isolatedsilanol groups but is substantially free of hydroxyl groups formed bysurface adsorbed water and surface silanol groups which are sufficientlyclosely spaced to be capable to interact by formation of hydrogenbridges.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferably the silica aerogel is a pyrogenically obtained aerogel suchas formed by the hydrolytic cleavage of silicon tetrachloride in a flameand subsequent coagulation to an aerogel. The time of treatment ispreferably between 1 and 60 seconds.

The fluidizing bed preferably is a vertical bed into which the silicondioxide and the gas are passed in countercurrent. The dry gas ispreferably preheated to a temperature between 100°and 300° C. andpreferably may be air.

Surprisingly, it can be experimentally shown that the silica aerogelobtained by the process of the invention and characterized by theabsence of bonded silanol groups and freedom of adsorbed water whileretaining the same or a smaller number of widely spaced free silanolgroups has a better thickening effect than a usual, or even waterfreeproduct which is obtained by a conventional pyrolytic process.

It has been found that these results can be obtained in pyrogenicallyobtained silicon dioxide aerogels and also in certain types ofnon-pyrogenically obtained silicic acids without application of reducedpressure and without extended periods of treatment. This result isobtained, as indicated above, by subjecting the silicon dioxide aerogelor another suitable type of silicic acid in a continuous apparatus tothe above-given temperatures for a very short time as indicated whilebeing exposed to a flow of dry gas. Preferably, the silica aerogel andthe gas mixture are passed into the apparatus in countercurrent andpreferably employing a vertical fluidizing bed.

It was particularly surprising that the removal of the water and thecondensation of part of the silanol groups in silicon dioxide aerogels,which can be condensed at these temperatures, proceeds at a much fasterpace and much more completely than in a partial vacuum. It wasfurthermore unexpected that, with such short time high-temperaturetreatment, the number of the more widely spaced free silanol groups isnot affected, as distinguished from any vacuum treatment but is retainedalmost 100% as can be proven by infrared spectra of the final products.This is of great significance since it is this feature which is at thebase of the technical value of the silicon dioxide aerogels asdistinguished from precipitated silicic acids in which the silanolgroups predominantly are closely spaced as so-called bonded silanolgroups and in which the less closely spaced, so-called free silanolgroups are of a comparatively insignificant amount.

It is also surprising that with the high-temperature treatment of theinvention the rehydration property is fully retained, which is directlycontrary to what has been reported in the literature in respect ofsilicic acids which had been subjected to calcination at such hightemperatures.

A further advantage of the process of the invention is that allcontaminations which are volatized at these temperatures, such as forinstance any residual hydrogen chloride, are completely eliminated.Thus, the advantages of an extended heat treatment at lower temperaturessurprisingly are retained with the short-time, high-temperaturetreatment of the invention while, on the other hand, the above-mentionedshortcomings of high temperatures are avoided, apparently because of thevery short residence time in the apparatus.

The thermal process of the invention can be directly joined to theprocess of making the silicon dioxide aerogel by means of a subsequentstage of operation. However, it is also possible to carry out theprocesses at a separate place and at a different time, for instance atthe place of use immediately prior to further processing of the product.In both cases, an absolutely dry silica aerogel is obtained from whichat the same time all acid has been removed.

The absence of bonded silanol groups and of adsorbed water make thisproduct particularly suitable where a highly active product is desired.

There is thus obtained by this process a novel product which is a finelydispersed, absolutely dry silicon dioxide characterized by the retentionof the free silanol groups and at the same time absence of hydroxylgroups of surface-adsorbed water and absence of or presence only in aninfinitesimal amount of such surface hydroxyl groups which are capableto interact together upon formation of hydrogen bridges.

The high degree of activity does not only concern the addition oradsorption of reactive materials but also chemical reactions takingplace with the silanol groups proceed in an easier and more completemanner with the product of the invention than with an untreated orpreviously dried aerogel. This high degree of activity also is expressedin chemical reactions with the reactive siloxane groups which are formedin substantial numbers by the process of the invention and which, on theone hand, similar to the silanol groups, are suitable for the cleavageof reactive materials and subsequent chemical reaction while, on theother hand, they are also fit for direct addition of, for instance,polar compounds such as alcohols or amines, etc.

The product of the present process can immediately be incorporated assuch into any desired organic solvent and can then be used for shippingin this manner as absolutely dry silica aerogel without the risk thathumidity enter this type of a paste during shipping or at the place offurther processing.

The process of the invention can be used in connection with differentkinds of, for instance, pyrogenically obtained silicon dioxide includingalso hydrophobized or otherwise surface-treated materials. All thesestarting products may thus be converted into silica aerogels of aparticularly high activity.

Thus, it was for instance possible to treat silica aerogels with asurface between 100 and 400 m² /g and having various contents of waterbetween 0.2 and 5.0%. Specific BET surface refers to the ratio of thetotal surface of the particle to its volume, which is the surface areaper unit of mass, as determined by the method of Brunauer, Emmett andTeller that is described in the Journal of the American ChemicalSociety, volume 60, page 309 (1938). All these materials could bemodified by the process of the invention so as to exhibit an increasedpH and also a change in their surface properties and a substantiallyincreased thickening action and thixotropy.

Surprisingly, even silica aerogels with organic contaminations furnishedhighly active, extremely pure aerogels by the process of the inventionthough treated for a time of only a few seconds.

The following examples will serve to further explain the invention.

EXAMPLE 1

The following materials were subjected to comparative tests:

a. an untreated silica aerogel with a surface of 380 m² /g;

b. the same type of aerogel which had been dried at 110° C. and10.sup.⁻³ Torr for a period of 11/2 hours;

c. the same type of silica aerogel as at (a) which had been heated at950° C. and 2 to 4 Torr in a tubular furnace for a period between 10 and25 seconds; and

d. again the same type of material as at (a) which had been treated fora time from 10 to 25 seconds at 950° C. and normal pressure incountercurrent with an oxygen-containing gas mixture that had beenpreheated to between 150° and 200° C.

Equal amounts of the materials obtained in all these four processes weresuspended in the same amount of carbon tetrachloride and sealed intotest tubes.

Even after a prolonged time the marked differences in thickening actioncould be clearly recognized. These observations could still be madeafter a storage time of 4 months.

In each of the four tests (a), (b), (c) and (d) there was an increasingthickening action from (a) to (d). In the test (a) the thickening actioncould be considered inadequate. In test (b) the action could beconsidered adequate, while in test (c) it could be designated as good,and in test (d) it could be designated as excellent.

For instance, in test (d) there was obtained a viscosity of 500 cp byforming a 5% suspension of the product in pure dry carbon tetrachlorideand measuring the viscosity with the "Rheometer" (stage I) after aperiod of 20 minutes.

EXAMPLE 2

Fresh silicon dioxide aerogel having a surface of 300 m² /g and a normalcontent of adsorbed, (physisorbed) water of about 0.5% was treated in atubular quartz furnace having a length of 1.20 m, a diameter of 25 mmand a heating zone of a length of 1 m. The material was subjected to thesame process as in Example 1, test (d). It was thus converted into anabsolutely dry, pure silica aerogel which was free of bonded silanolgroups, as could be proven by the infrared spectrum of the startingproduct compared with the final product.

The product obtained in the infrared spectrum showed a single narrowstrong band at about 3700 cm.sup.⁻¹ within the range between 4000 and2000 cm.sup.⁻¹.

In a continuous operation, there were obtained, depending on the input,500 to 800 g per hour of the dry product. The amount of dry gas employedwas between 50 and 100 l/h of air which had been preheated to between100° and 200° C. and was passed through the silica material incountercurrent.

EXAMPLE 3

In this case a silica aerogel was treated which had an initial pH of2.55. The same treatment was applied as in Example 1, test (d). After aresidence time of 1 second, the pH had risen to 3.30 and after anothersecond the deacidification had proceeded to a pH of 3.85.

The final product was again an absolutely dry silica aerogel asdescribed above.

EXAMPLE 4

The initial material in this case was a silica aerogel with a surface of170 m² /g and a pH of 2.50. The material was treated by the processdescribed in Example 1, test (d), with a residence time of between 2 and5 seconds. After the treatment the material had a pH of 3.50.

The viscosity of the material was measured as described in Example 1,test (d) and was found to be in excess of 480 cp, while the viscosity ofthe starting product was found to be 384 cp.

EXAMPLE 5

A hydrophobized silica aerogel was treated as described in Example 1,test (d). Continuous operation with an hourly throughput of 500 gresulted in a product in which no carbon could be detected.

EXAMPLE 6

This example illustrates the high activity of the product obtained.

A silicon dioxide which had been treated as described in Example 1, test(d) was charged at 20° C. with methanol vapor by exposing it for fourhours to a saturation vapor pressure of methanol at the temperatureindicated while excluding all other gases. The obtained product was thensubjected for half an hour to a temperature of 100° C. in a vacuum of10.sup.⁻³ Torr. Analysis showed a carbon content of 0.4%.

For comparison, the same type of silicon dioxide which, however, had notundergone the short-time heat treatment, after drying of the adsorbedwater at 100° C. was treated exactly as described above with methanolvapor and heating. It had a carbon content of only 0.02%.

EXAMPLE 7

This example illustrates a treatment at a time remote from the time ofmaking the silicon dioxide.

A silica aerogel ("Aerosil") which had been stored for about 2 years wassubjected in this case to the treatment of the invention. The aerogelhad a surface area of 200 m² /g and a water content (adsorbed,physisorbed water) of about 2%. The material was treated in a tubularquartz furnace of a length of 1200 mm and an active heating area of 1000mm length and an internal diameter cf 100 mm. The furnace was heated toa temperature between 900° and 1100° C. Otherwise, the type of treatmentwas the same as in Example 1, test (d).

There was obtained in this process an absolutely dry pure silica aerogelwhich was absolutely free of bonded silanol groups as could be shown bya comparison of the infrared spectra of the starting product and thefinal product. The final product in the infrared spectrum between 4000cm.sup.⁻¹ and 2000 cm.sup.⁻¹ showed only one single sharp intense bandat about 3700 cm.sup.⁻¹.

In a continuous process there were obtained 10 kg of the dry product in1 hour. The gas employed was dry air which had been heated to 300° C.and which was passed through the silica aerogel in countercurrent in anamount of about 4 m³ /h.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. A highly active superdry silicaaerogel free of adsorbed water and hydrogen-bonded silanol groups theinfrared spectrum of which is characterized by a single narrow intenseband at about 3700 cm.sup.⁻¹ within the range between 4000 and 2000cm.sup.⁻¹ indicating the presence of free silanol groups and whichspectrum is further characterized by the absence of bonds indicating thepresence of adsorbed water and hydrogen-bonded silanol groups, the saidsilica aerogel having been produced by subjecting a pyrogenicallyproduced silica aerogel containing adsorbed water and both free andhydrogen-bonded silanol groups to treatment in a fluidized bed at atemperature between about 700° and 1000° C. and at a pressure between 2torrs and atmospheric pressure for a period between 1 second and a fewminutes through which fluidized bed a stream of dried gas that had beenpreheated to a temperature between 100° and 300° C. was passed.
 2. Ahighly-active super-dry silica aerogel free of adsorbed water andhydrogen-bonded silanol groups, the infrared spectrum of which ischaracterized by a single narrow intense band at about 3700 cm.sup.⁻¹within a range between 4000 and 2000 cm.sup.⁻¹ indicating the presenceof free silanol groups and which spectrum is further characterized bythe absence of bonds indicating the presence of adsorbed water andhydrogen-bonded silanol groups, said silica aerogel having a surfacebetween 100 and 400 m² /g.
 3. A highly-active super-dry silica aerogelaccording to claim 2 wherein the silica is pyrogenic silica.
 4. Thepyrogenic silicon dioxide of claim 3 on the surface of which all of thewidely spaced silanol groups are present which are present also on thesurface of a pyrogenic silicon dioxide where additionally-adsorbed wateror hydrogen-bonded silanol groups, or both said water and bonded silanolgroups, are present.